1. Field of the Invention
The present invention relates to an organic electroluminescent element, particularly, to an organic electroluminescent element comprising an improved organic hole transfer layer containing a polymer type organic hole transfer material.
2. Description of the Related Art
The organic electroluminescent element comprises an anode, a cathode and an organic light-emitting layer interposed between the anode and the cathode and exhibiting an electric conductivity and a light-emitting capability. If voltage is applied to the organic electroluminescent element in the forward direction, holes are injected from the anode into the organic light-emitting layer and electrons are injected from the cathode into the organic light-emitting layer. The organic electroluminescent element is a spontaneous light-emitting element, in which these holes and electrons are recombined within the organic light-emitting layer so as to form excitons, and then light is emitted when the excitons are relaxed. Since the organic electroluminescent element permits the process for forming a large display area having a low temperature, the organic electroluminescent element is expected to be applied to a thin film display of the next era.
In such an organic electroluminescent element, the injection of electrons and holes from the electrodes and the mobility of the electrons and holes are controlled by arranging a plurality of layers such as a hole injection layer, a hole transfer layer, an electron transfer layer and an electron injection layer between the anode and the cathode in addition to the organic light-emitting layer. Japanese Patent Application Kokai Publication No. 2004-63408 discloses a hole transfer layer formed of a polymer type hole transfer material such as a so-called “PEDOT:PSS”, which is prepared by having poly (ethylenedioxy) thiophene (DEDOT) doped with polystyrenesulphonic acid (PSS). If the hole transfer layer is interposed between the anode and the organic light-emitting layer, it is possible to lower the barrier wall for the hole injection, compared with the case where the holes are injected from the anode formed of a transparent electrode such as ITO directly into the organic light-emitting layer. For example, the barrier wall for the hole injection can be lowered from about 1.0 V to about 0.5 V in the case of forming a hole transfer layer between the anode and the organic light-emitting layer.
However, the organic electroluminescent element comprising the hole transfer layer is defective in that the organic light-emitting layer is deteriorated by the hole transfer layer in the current driving stage so as to shorten the life of the organic electroluminescent element.